Reduction of Electromagnetic Interference in a Capacitive Touchscreen System

ABSTRACT

Disclosed herein are various embodiments of circuits and methods for reducing electromagnetic interference in mutual capacitance measurement or sensing systems, devices, components and methods such as capacitive touchscreens. Charge integrator circuits and switched capacitance filtering circuits are disclosed that improve the signal-to-noise ratio (or the ratio of a desired sensed mutual capacitance signal to an undesired EMI signal) in a capacitive sensor readout circuit without the need to increase the amplitude of the drive signal. The various embodiments of the charge integrator and switched capacitance filtering circuits described herein permit an improvement in noise immunity without requiring the excessive power levels typically associated with high amplitude drive circuitry, and moreover result in boosting signal-to-noise ratios during early stages of signal processing.

FIELD OF THE INVENTION

Various embodiments of the invention described herein relate to thefield of capacitive sensing devices generally, and more specifically tocircuits and methods for reducing or filtering electromagneticinterference in mutual capacitance measurement or sensing systems,devices, components and methods such as capacitive touchscreens.

BACKGROUND

Two principal capacitive sensing and measurement technologies are,currently employed in most touchpad and touchscreen devices. The firstsuch technology is that of self-capacitance. Many devices manufacturedby SYNAPTICS™ employ self-capacitance measurement techniques, as dointegrated circuit (IC) devices such as the CYPRESS PSOC.™Self-capacitance involves measuring the self-capacitance of a series ofelectrode pads using techniques such as those described in U.S. Pat. No.5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld ComputingDevice” dated Aug. 6, 1996.

Self-capacitance may be measured-through the detection of the amount ofcharge accumulated on an object held at a givens voltage (Q=CV).Self-capacitance is typically measured by applying a known voltage to anelectrode, and then using a circuit to measure how much charge flows tothat same electrode. When external objects are brought close to theelectrode, additional charge is attracted to the electrode. As a result,the self-capacitance of the electrode increases. Many touch sensors areconfigured such that the grounded, object is a finger. The human body isessentially a capacitor toe surface where the electric field vanishes,and typically has a capacitance of around 100 pF.

The second primary capacitive sensing and measurement technologyemployed in touchpad and touchscreen devices is that of mutualcapacitance, where measurements are performed using a crossed grid ofelectrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheideentitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999.Mutual capacitance technology is employed in touchpad devicesmanufactured by CIRQUE.™ In mutual capacitance measurement, capacitanceis measured between two conductors, as opposed to a self-capacitancemeasurement in which the capacitance of a single conductor is measured,and which may be affected by other objects in proximity thereto.

In a capacitive touchscreen, a user's finger represents an electrodeconnected to an electric field ground. Due to the widespread use ofswitching power supply converters to power capacitive touchscreens, theelectric potential of a readout electronic ground terminal (orsystem-ground) may vary significantly in periodic or not strictlyperiodic fashion with respect to the voltage associated with theelectric field ground. The electric field voltage or potential isassumed to be zero at infinity. Variation of the system ground voltagewith respect to the electric field ground voltage may producesignificant interference in mutual capacitance signals acquired from acapacitive touchscreen, which we refer to here generically aselectromagnetic interference. Such interference. Can be considered as“noise” due to the generally unknown phase relationships between areadout sampling clock a swathing power supply converter block. Othercontributions to electromagnetic interference can include “noise”coupling associated with undesired charges induced in the human body andthe touchscreen by various ambient or environmental sources that may beasynchronous with respect to capacitive touchscreen drive signals.

What is needed is a capacitive touchscreen system capable of reducing orotherwise filtering the effects of electromagnetic interference.

SUMMARY

In one embodiment, there is, a provided a capacitive touchscreen systemcomprising a touchscreen comprising a first plurality of electricallyconductive drive electrodes arranged in rows or columns, and a secondplurality of electrically conductive sense electrodes arranged in rowsor columns arranged at an angle with respect to the rows or columns ofthe first plurality of electrodes, mutual capacitances existing betweenthe first and second pluralities of electrodes at locations where thefirst and second pluralities of electrodes intersect, such mutualcapacitances changing in the presence of one or more fingers or touchdevisees brought into proximity thereto, at least one drive circuitoperably connected to the first plurality of electrodes and configuredto provide drive signals of high and low states thereto, a plurality ofsense circuits, one each of the sense circuits being operably connectedto corresponding ones of the second plurality of electrodes, a systemground associated with the capacitive touchscreen system, anelectric-field ground associated with the one or more fingers or touchdevices and the touchscreen, wherein each of the sense circuitscomprises a charge integrator circuit and a switched capacitancefiltering circuit, the charge integrator circuit being configured toreceive input signals provided by a corresponding one of the secondplurality of electrodes and to provide integrated signals to theswitched capacitance filtering circuit, the switched capacitancefiltering circuit being configured to sample and store in first andsecond capacitors, respectively, first and second integrated signalsdelivered thereto by the charge integrator circuit where the firstintegrated signal corresponds to the high state drive signal, and thesecond integrated signal corresponds to the low state drive signal, theswitched capacitance filtering circuit being configured to provide anoutput signal representative of the mutual capacitance of the one of thesecond plurality of electrodes corresponding to the sense circuit andfurther having an electromagnetic interference voltage between thesystem ground and the electric field ground substantially filteredtherefrom by the switched capacitance filtering circuit.

In another embodiment, there is provided a processor for a capacitivetouchscreen system comprising a touchscreen comprising a first pluralityof electrically conductive drive electrodes arranged in rows or columns,and a second plurality of electrically conductive sense electrodesarranged in rows or columns arranged at an angle with respect to therows or columns of the first plurality of electrodes, mutualcapacitances existing between the first and second pluralities ofelectrodes at locations where the first and second pluralities, ofelectrodes intersect, such mutual capacitances changing in the presenceof one or more fingers or touch devices brought into proximity thereto,a system ground being associated with the capacitive touchscreen system,an electric field ground being associated with the one or more fingersor touch devices and the touchscreen, the processor comprising at leastone drive circuit operably connected to the first plurality ofelectrodes and configured to provide drive signals of high and lowstates thereto, a plurality of sense circuits, one each of the sensecircuits being operably connected to corresponding ones of the secondplurality of electrodes, wherein each of the sense circuits comprises acharge integrator circuit and a switched capacitance filtering circuitthe charge integrator circuit being configured to receive input signalsprovided by a corresponding one of the second plurality of electrodesand to provide integrated signals to the switched capacitance filteringcircuit, the switched capacitance filtering circuit being configured tosample and store in first and second capacitors, respectively, first andsecond integrated signals delivered thereto by the charge integratorcircuit, where the first integrated signal corresponds to the high statedrive signal, and the second integrated signal corresponds to the lowstate drive signal, the switched capacitance filtering circuit beingconfigured to provide an output signal representative, of the mutualcapacitance of the one of the second plurality of electrodescorresponding to the sense circuit and further having an electromagneticinterference voltage between the system ground and the electric fieldground substantially filtered therefrom by, the switched capacitancefiltering circuit.

In still another embodiment there is provided a method of reducingelectromagnetic interference in a capacitive touchscreen systemcomprising a touchscreen comprising a first plurality of electricallyconductive drive electrodes arranged in rows or columns, and a secondplurality of electrically conductive sense electrodes arranged in rowsor columns arranged at an angle with respect to the rows or columns ofthe first plurality of electrodes, mutual capacitances existing betweenthe first and second pluralities of electrodes at locations where thefirst and second pluralities of electrodes intersect, such mutualcapacitances changing in the presence of one or more fingers or touchdevices brought into proximity thereto, a system ground being associatedwith the capacitive touchscreen system, an electric field ground beingassociated with the one or more fingers or touch devices and thetouchscreen, the method comprising sequentially driving the firstplurality of electrodes with alternating high and low state drivesignals, sensing the mutual capacitances associated with each of thesecond plurality of electrodes during the high and low state drivesignals with a charge integrator circuit for each sense circuit, foreach charge integrator circuit, providing first and second integratedsignals corresponding to the high and low state drive signals,respectively, to a switched capacitance filtering circuit, sampling andstoring in first and, second capacitors, respectively, of the switchedcapacitance filtering circuit the first and second integrated signals,providing, with the switched capacitance filtering circuit, an outputsignal representative of the mutual capacitance of the one of the secondplurality of electrodes corresponding to the sense circuit and furtherhaving an electromagnetic interference voltage between the system groundand the electric field ground substantially filtered therefrom by theswitched capacitance filtering circuit.

Further embodiments are disclosed herein or will become apparent tothose skilled in the art after having read and understood thespecification and drawings hereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Different aspects of the various embodiments of the invention willbecome apparent from the following specification, drawings and claims inwhich:

FIG. 1 shows a cross-sectional view of one embodiment of a capacitivetouchscreen system;

FIG. 2 shows a block diagram of a capacitive touchscreen controller;

FIG. 3 shows one embodiment of a block diagram of a capacitivetouchscreen system and a host controller;

FIG. 4 shows a schematic block diagram of one embodiment of a capacitivelei touchscreen system;

FIG. 5 shows one embodiment of a, touch sensor pixel circuit in thepresence of a finger touch;

FIG. 6 shows one embodiment off-a touch sensor pixel circuit in theabsence of a finger touch;

FIG. 7 shows a schematic illustration of a touch sensor pixel circuitwith a charge integrator circuit;

FIG. 8 shows one embodiment of a switched capacitance filtering circuit;

FIG. 9 shows one embodiment of a timing control sequence for the circuitof FIG. 8;

FIG. 10 shows circuit simulation and analytical results obtained atlower frequencies for one embodiment of a charge integrator circuit andcorresponding switched capacitance-filtering circuit;

FIG. 11 shows circuit simulation and analytical results obtained at awider range of frequencies for one embodiment of a charge integratorcircuit and corresponding switched, capacitance filtering circuit;

FIG. 12 shows another embodiment of a switched capacitance filteringcircuit;

FIG. 13 shows still another embodiment of a switched capacitancefiltering circuit, and

FIG. 14 shows analytical results obtained for three differentembodiments of a charge integrator circuit and corresponding switchedcapacitance filtering circuit.

The drawings are not necessarily to scale. Like numbers refer to likeparts or steps throughout the drawings.

DETAILED DESCRIPTIONS OF SOME EMBODIMENTS

As illustrated in FIG. 1, a capacitive touchscreen system 110 typicallyconsists of an underlying LCD or OLED display 112, an overlyingtouch-sensitive panel or touchscreen 90, a protective cover ordielectric plate 95 disposed over the touchscreen 90, and a touchscreencontroller, microprocessor, application specific integrated circuit(“ASIC”) or CPU 100. Note that image displays other than LCDs or OLEDsmay be disposed beneath display 112.

FIG. 2 shows a block diagram of one embodiment of a touchscreencontroller 100. In one embodiment, touchscreen controller 100 may be anAvago Technologies™ AMRI-5000 ASIC or chip 100 modified in accordancewith the teachings presented herein. In one embodiment, touchscreencontroller is a low-power capacitive touch-panel controller designed toprovide a touchscreen system with high-accuracy, on-screen navigation.

Capacitive touchscreens or touch panels 90 shown in FIGS. 3 and 4 can beformed by applying a conductive material such as Indium Tin Oxide (ITO)to the surface(s) of a dielectric plate, which typically comprisesglass, plastic or another suitable electrically insulative andpreferably optically transmissive material, and which is usuallyconfigured in the shape of an electrode grid. The capacitance of thegrid holds an electrical charge, and touching the panel with a fingerpresents a circuit path to the uses body, which causes a change in thecapacitance.

Touchscreen controller 100 senses and analyzes the coordinates of thesechanges in capacitance. When touchscreen 90 is affixed to a display witha graphical user interface, on-screen navigation is possible by trackingthe touch coordinates. Often it is necessary to detect multiple touches.The size of the grid is driven by the desired resolution of the touches.Typically there is an additional cover plate 95 to protect the top ITOlayer of touchscreen 90 to form a complete touch screen solution (see,e.g., FIG. 1).

One way to create a touchscreen 90 is to apply an ITO grid on one sideonly of a dielectric plate or substrate. When the touchscreen 90 ismated with a display there is no need for an additional protectivecover. This has the benefit of creating a thinner display system withimproved transmissivity (>90%), enabling brighter and lighter handhelddevices. Applications for touchscreen controller 100 include, but arenot limited to, smart phones, portable media players, mobile internetdevices (MIDs), and GPS devices.

Referring now to FIGS. 3 and 4, in one embodiment the touchscreencontroller 100 includes an analog front end with 9 sense and drivesignal lines and 16 drive and sense lines connected to an ITO grid on atouchscreen. Touchscreen controller 100 applies an excitation such as asquare wave, meander signal or other suitable type of drive signal tothe drive electrodes that may have a is frequency selected from a rangebetween about 40 kHz and about 200 kHz. The AC signal is coupled to thesense lines via mutual capacitance. Touching panel 90 with a fingeralters the capacitance at the location of the touch. Touchscreencontroller 100 can resolve and track multiple touches simultaneously. Ahigh refresh rate allows the host to track rapid touches and anyadditional movements without appreciable delay. The embedded processorfilters the data, identifies the touch coordinates and reports them tothe host. The embedded firmware can be updated via patch loading. Othernumbers of drive and sense lines are of course contemplated, such as8×12 and 12×20 arrays.

According to one embodiment, and as shown in FIG. 4, an ITO grid can beemployed on touchscreen 90, and comprises rows 20 a-20 p and columns 10a-10 i, where rows 20 a-20 p are operably connected to sense circuitsand columns 10 a-101 are operably connected to drive circuits. FIG. 4illustrates one configuration for routing ITO or other drive and senselines to touchscreen controller 100. Rows and columns for sense anddrive lines are interchangeable.

Those skilled in the art will understand that touchscreen controllers,micro-processors, ASICs or CPUs other than a modified AMRI-5000 chip ortouchscreen controller 100 may be employed in touchscreen system 110,and that different numbers of drive and sense lines, and differentnumbers and configurations of drive and sense electrodes, other thanthose explicitly shown herein may be employed without departing from thescope or spirit of the various embodiments of the invention.

Referring now to FIGS. 5 and 6, there are shown capacitive networkcircuits 50 and 52 representing an individual pixel sensor in a mutualcapacitance touchscreen, where in FIG. 5 the touchscreen is subjected toa finger touch by a user, and in FIG. 6 the touchscreen is not touchedby a user. A typical touchscreen comprises many such circuits, each pairof circuits representing intersections of sense and drive lines. In atouched pixel sensor, a finger electrode is connected to an electricfield ground by a human body capacitor C_(hb), which typically has avalue of about 100 pF. The circuit models shown in FIGS. 5 and 6 areshown in the context of two types of electrodes disposed along rows andcolumns, X and Y axes, or any other suitable arrangement of drive andsense electrodes. For purposes of clarity and illustration, FIGS. 5 and6 show the various types of capacitances associated with such axes withX and Y designations.

The various types of capacitances shown in FIGS. 5 and 6 are nowdescribed. In FIG. 5 (the “touched” case), C_(xyt1) represents electrodeoverlap capacitance in the presence of a finger touch, C_(xfgt1)represents X electrode to electric field ground capacitance in thepresence of a finger touch, C_(yfgt1) represents Y electrode to electricfield ground capacitance in the presence of a finger touch, C_(fx1)represents finger to X electrode capacitance, C_(fy1) represents fingerto Y electrode capacitance, and C_(hb) represents human bodycapacitance. In FIG. 6, C_(xyn) represents X-Y electrode overlapcapacitance in absence of a finger touch, C_(xfgn) represents Xelectrode to field ground capacitance in the absence of a finger touch,and C_(yfgn) represents Y electrode to field ground capacitance in theabsence of a finger touch. As will be seen by referring to FIGS. 5 and6, the various capacitance designations are similar.

Note that pixel resistivity is not taken in to account in the circuitmodels of FIGS. 5 and 6. Neglecting resistivity in such circuit modelsis valid for touchscreen operation at appropriate frequencies orcharging times, which by way of example may range between about 3microseconds and about 100 microseconds. Such appropriate frequencies orcharging times permit sufficient time for the completion of chargeredistribution in the touchscreen and the associated acquisition orsensing electronics each time after drive signal potentials have changedvalue. To describe touchscreen resistivity effects, the capacitancesassociated with each touch sensor pixel should be connected to terminalswith resistors rather then ideal conductors. In one embodiment, onedrive electrode from among a plurality of drive electrodes in thetouchscreen is connected to a voltage source. Sense electrodes of thetouchscreen are connected to sensor readout electronics (or acquisitionor sense circuitry).

In a capacitive touchscreen, a user's finger represents an electrodeconnected to an electric field ground. Due to the widespread use ofswitching power supply converters to power capacitive touchscreens, theelectric potential of a readout electronic ground terminal (or systemground) may vary significantly in periodic or not strictly periodicfashion with respect to the voltage associated with the electric fieldground. The electric field voltage or potential is assumed to be zero atinfinity. Variation of the system ground voltage with respect to theelectric field ground voltage may produce significant interference inmutual capacitance signals acquired from a capacitive touchscreen, whichwe refer to here generically as electromagnetic interference. Suchinterference may be considered as “noise” due to the generally unknownphase relationships between a readout sampling clock a switching powersupply converter clock. Other contributions to electromagneticinterference can include “noise” coupling associated with undesiredcharges induced by various ambient or environmental sources that may beasynchronous with respect to capacitive touchscreen drive signals.

FIG. 7 shows a circuit model representing a single touch sensor pixeltouched by a human finger. The pixel is charged by a voltage drop V_(d)between the drive electrode and the system ground. Note that theelectric field ground is designated as a solid black triangle. Oneterminal of human body capacitor is connected to the electric fieldground and the other terminal is connected to the finger electrode.Electromagnetic interference is described by the voltage source V_(emi)connected between the system ground and the electric field ground. Uponreset of charge integrator circuit 61, initial conditions areestablished at charge integrator circuit 61's virtual ground, andelectric charge is collected in feedback capacitor C_(f), which isinduced by charging circuit 60 by voltage sources V_(d) and V_(emi). Anexpression for the ratio of the charges caused by charging capacitor C₁by voltage sources V_(d) and V_(emi); is shown in equation (10 below,where the designations of FIGS. 5 and 6 are employed, and thecapacitance of the touch sensor pixel in touched and untouched states isrepresented by C_(dst) and C_(dsnt), respectively. Note that C_(sf)represents sense electrode to finger capacitance in a touched state.

$\begin{matrix}{\frac{Q_{signal}}{Q_{gemi}} \cong \frac{V_{d}( {C_{dsnt} - C_{dst}} )}{V_{gemi}C_{sf}}} & (1)\end{matrix}$

Equation (1) shows that the signal-to-noise ratio (where electromagneticinterference or “EMI” represents noise) is directly proportional to, theratio of the drive and EMI signal amplitudes. As a result, thesignal-to-noise ratio many be improved by increasing the amplitude ofthe drive signal. Below are described means and methods for effectingelectronic readout through the sense electrodes and correspondingacquisition or sensing circuitry where the signal-to-noise ratio isimproved without increasing the amplitude of the drive signal. Thesetechniques increase noise immunity without dissipating excessive powerthrough the use of high amplitude drive circuitry.

Owing to the superposition principle, the drive electrode voltage V_(d)and the system ground voltage V_(emi) both independently contribute tothe output signal provided by charge integrator circuit 61. Thesecontributions are uncorrelated provided V_(d) and V_(emi) areuncorrelated. The signals sampled at the output of charge integratorcircuit 61 occurring at certain repeated voltage values provided by thedrive electrode contain approximately equal electromagnetic interferencesignal contributions, so long as the sample interval is much shorterthan variation of the electromagnetic interference signals over time.

A circuit configured to sample the output signal of charge integratorcircuit 61 for equal charging times with the voltage V_(d) being held inalternating high and low states (V_(drh) and V_(drl), respectively) isnow discussed. Referring to FIGS. 5 and 6, and neglecting the limitedopen loop gain of the charge integrator circuit amplifier, an expressionfor the signal-to-noise ratio at the output of charge integrator circuit61 is derived as follows:

$\begin{matrix}{( \frac{S}{N} )_{t} \cong \frac{{{V_{drh} - V_{drl}}}( {C_{{xyt}\; 1} + \frac{C_{{fx}\; 1}C_{{fy}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}{V_{{emi\_ pk} - {pk}}( {C_{{xfgt}\; 1} + \frac{C_{hb}C_{{fx}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}} & (2)\end{matrix}$

where the designation for the EMI amplitude peak to peak value isV_(emi) _(—) _(pk-pk). The numerator of equation (2) defines a chargedifference caused by the drive potential V_(d) in high and low states.The denominator of equation (2) defines the difference between minimumand maximum collected by charge integrator circuit 61 due to EMI. Thetransfer function of charge integrator circuit 61 is the same for boththe EMI harmonic and drive signals, and thus drops out of equation (2).

Under no touch conditions a similar expression for the signal-to-noiseratio using the designations of FIGS. 5 and 6 may be obtained:

$\begin{matrix}{( \frac{S}{N} )_{n} \cong \frac{{{V_{drh} - V_{drl}}}C_{xyn}}{V_{{emi\_ pk} - {pk}}C_{xfgn}}} & (3)\end{matrix}$

One embodiment of a switched capacitance filtering circuit 62 configuredto perform correlated sampling is shown in FIG. 8; where the symbol ‘+’designates a logical OR operation for switching circuitry controlsignals. At different levels of the drive signal amplitude (V_(drn) andV_(drl)), capacitors C₁ and C₂ in switched capacitance filtering circuit62 are charged by the signal provided by the output of charge integratorcircuit 61 of FIG. 7. The command sequence used to control switchedcapacitance filtering circuit 62 is shown in FIG. 9. Note that switchesS₁ and S₂ are closed during the logical high state.

One expression describing the EMI signal spectrum after the correlatedsampling circuit has performed its operations is as follows:

V _(emi) _(—) _(cs)(ω)=V _(emi) _(—) _(out)(ω)exp[−jωΔt]−V _(emi) _(—)_(out)(ω)  (4)

where Δt is the time interval between samples, and V_(emi) _(—) _(out)is the EMI signal presented at the output of charge integrator circuit61

When equation (2) is extended to include the effects of correlatedsampling in the frequency domain, it becomes equation (5):

$\begin{matrix}{( \frac{S}{N} )_{tcs} = \frac{{{V_{drh} - V_{drl}}}( {C_{{xyt}\; 1} + \frac{C_{{fx}\; 1}C_{{fy}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}{2V_{{emi}\; 0}{{\sin \lbrack \frac{\pi \; f_{0}}{( {{1/\Delta}\; t} )} \rbrack}}( {C_{{xfgt}\; 1} + \frac{C_{hb}C_{{fx}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}} & (5)\end{matrix}$

where V_(emi0) is the amplitude of the EMI harmonic at terminal ‘f_(g)’terminal, and f₀ is the harmonic oscillation frequency. While thesignal-to-noise ratio of equation (5) is still proportional to the ratioof the drive signal amplitude differences with respect to the EMIamplitude peak-to-peak amplitude differences, it now includes the ratioof the harmonic frequency to the inverse time intervals between samplesdescribed by a sine function. The filtering property of this additionalterm is used to attenuate EMI harmonic noise at certain frequencyranges, and thus permits filtering of EMI.

FIG. 10 shows simulation results obtained using circuits 50 and 52 ofFIGS. 5 and 6, respectively, where the pixel circuits of FIGS. 5 and 6were driven by a voltage source connected to ‘Y’ pixel terminal andsensed or read with a charge integrator circuit connected to an ‘X’pixel terminal, followed by the output of charge integrator circuit 61being provided to switched capacitance filtering circuit 62 of FIG. 8.The electric field ground (f_(g)) terminal (or the second plate of humanbody self capacitance) was connected to system ground through a voltagesource V_(emi), which generated harmonic oscillations at frequenciesrepresenting the EMI signal. The sensing or readout circuit included aswitched capacitance filtering circuit similar to that shown in FIG. 8.An ideal operational amplifier with an open loop gain of 10K was used incharge integrator circuit 61, while charge integrator circuit feedbackcapacitor C_(f) was assigned a value of 10 pF. Ideal switches wereclosed when logical high signals were applied to the switch driveterminals. The virtual ground of charge integrator circuit 61 wasconnected to a voltage source of 0.9V. Other simulation parameters wereas follows: C_(xyn)=1.13 pF; C_(xyt1)=0.876 pF; C_(fy1)=1.396 pF;C_(fx1)=1.132 pF; C_(hb)=100 pF. Switches S₁ and S₂ permitted chargingof the sampling capacitors in switched capacitance filtering circuit 62using voltages provided at the output of charge integrator circuit 61having equally spaced timing t_(ch) associated with rising from V_(drl)to V_(drh) and falling from V_(drl) to V_(drh) at the respective edgesof the pulses associated with drive voltage V_(d). The samplingintervals employed were of the same length for each capacitor (0.9μsec.). Switching to the hold mode occurred before the drive potentiallevel changed value. The resulting difference between the signals heldin the capacitors of switched capacitance filtering circuit 62 waspresented to output thereof for further processing when the logical highof the S_(c) signal occurred. Note that the S_(c) signal was also usedto reset charge integrator circuit 61. The drive signal was configuredto change between V_(drl) and V_(drh)=1.8V.

Referring to FIG. 10, it will be seen that the obtained simulatorresults (represented by the solid black dot data points of FIG. 10) arein excellent agreement with the analytical results derived from equation(5) (represented by the solid continuous curve of FIG. 10) in equation(6) as follows:

$\begin{matrix}{( \frac{N}{S} )_{nor} = {\frac{{V_{{ripple\_ pk} - {pk}}/} < V_{out} >}{V_{{emi\_ pk} - {pk}}/{{V_{drh} - V_{drl}}}} = \frac{2{{\sin \lbrack \frac{\pi \; f_{0}}{( {{1/\Delta}\; t} )} \rbrack}}( {C_{{xfgt}\; 1} + \frac{C_{hb}C_{{fx}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}{( {C_{{xyt}\; 1} + \frac{C_{{fx}\; 1}C_{{fy}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}}} & (6)\end{matrix}$

where the right part of equation (6) represents the normalized harmonicEMI attenuation obtained using equation (5) and plotted in FIG. 10 as acontinuous curve, while simulated circuit results using time domainSPECTRE™ program simulations are shown in the solid black dots. Owing tothe filtering resulting from the correlated sampling effected byswitched capacitance filtering circuit 62, the EMI contribution atfrequencies substantially lower than f_(sam) (or 1/Δt) is attenuated asshown in FIG. 10. The resulting EMI harmonic attenuation was alsodiscovered to be substantially phase independent. FIG. 11 shows anexpanded plot, over a wider range of frequencies, of the simulation andanalytical results shown in FIG. 10. FIG. 11 shows that the transferfunction of switched capacitance filtering circuit 62 is similar to acomb notch filter, with filtering-properties governed by the timeintervals between samples.

Modified versions of switched capacitance filtering circuit 62 of FIG.8, switched capacitance filtering circuits 64, are shown in FIGS. 12 and13, where additional pairs of capacitors are employed to filter EMI.These additional pairs of capacitors permit adjustment of the resultingfiltering transfer function at frequencies that are both lower andhigher than the inverse time interval between adjoining samples. FIG. 12illustrates a switched capacitance filtering circuit 64 having N pairsof capacitors arranged in parallel with respect to one another. FIG. 13illustrates a switched capacitance filtering circuit 64 having threepairs of capacitors arranged in parallel with respect to one another.

A switched capacitance filtering circuit which combines correlatedsampling with signal averaging at the output thereof permits improvedEMI attenuation at certain frequencies, which may be controlled by theparameters of the sensing or acquisition circuit. The principal suchfiltering parameters are N, T/Δt, and Δt, more about which is now said.Equation (7) shown below is derived in a similar fashion to equation (6)for the case where multiple samples are averaged at the outputs ofswitched capacitance filtering circuits 64 shown in FIGS. 12 and 13.Note that with respect to equation (6), equation (7) contains twoadditional parameters: N, or the number of pairs of sampled signaldifferences, and T/Δt, the ratio of sample difference repetition timesto the time interval between the samples. Equation (7), then, is asfollows:

$\begin{matrix}{( \frac{N}{S} )_{nor} = {{\frac{2{\sin \lbrack \frac{\pi \; f_{0}}{f_{sam}} \rbrack}{\sin \lbrack {\frac{\pi \; f_{0}}{f_{sam}}\frac{T}{\Delta \; t}N} \rbrack}}{N\; {\sin \lbrack {\frac{\pi \; f_{0}}{f_{sam}}\frac{T}{\Delta \; t}} \rbrack}}}\frac{( {C_{{xfgt}\; 1} + \frac{C_{hb}C_{{fx}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}{( {C_{{xyt}\; 1} + \frac{C_{{fx}\; 1}C_{{fy}\; 1}}{C_{{fx}\; 1} + C_{{fy}\; 1} + C_{hb}}} )}}} & (7)\end{matrix}$

By increasing N, the number of notches in the transfer function isincreased. Varying T/Δt permits the range of frequencies over whichfiltering occurs to be matched to the panel charge time (e.g., 3 to 100microseconds). The time interval between samples, Δt, can be varied topermit lower or higher bandwidths of signals to be filtered.

to FIG. 14 shows analytical transfer function results derived fromequation (7) for switched capacitance filtering circuit 64 having onepair of capacitors (represented by curve 75), two pairs of capacitors(represented, by curve 77), and three pairs of capacitors (representedby curve 79). The simulated results shown in FIG. 14 were obtained usingcontrol time sequence parameters corresponding to Δt=6 μsec. andT/Δt=2.5, with a drive cycle period of 15 μsec.

It will now be seen that charge integrator circuit 61 in combinationwith switched capacitance filtering circuits 62 or 64 results in animproved signal-to-noise ratio (or ratio of the desired sensed mutualcapacitance signal to the undesired EMI signal) in a capacitive sensorreadout circuit without the need to increase the amplitude of the drivesignal. The various embodiments of the charge integrator and switchedcapacitance filtering circuits described herein permit an improvement innoise immunity without requiring the excessive power levels typicallyassociated with high amplitude drive circuitry, and moreover result inboosting signal-to-noise ratios during early stages of signalprocessing.

In one embodiment, and as shown in switched capacitance filteringcircuit 62 of FIG. 8, first and second capacitors C₁ and C₂ areelectrically connected in series, and switched capacitance filteringcircuit 62 comprises first and second input switches S₁ and S₂corresponding to the first and second capacitors, respectively. Firstinput switch S₁ is configured to close and charge first capacitor C₁when the first integrated signal corresponding to the high state drivesignal is delivered thereto while second switch S₂ is open. Second inputswitch S₂ is configured to close and charge second capacitor C₂ when thesecond integrated signal corresponding to the low state drive signal isdelivered thereto while first switch S₁ is open. A reference voltageV_(r) is switchably and operably connected between first and secondcapacitors C₁ and C₂ by a third switch (designated by S₁+S₂, which is acontrol signal). Such third switch is configured to close to providereference voltage V, to switched capacitance filtering circuit 62 whenfirst switch S₁ is closed, or when second switch S₂ is closed. Note thatsuch third switch must be closed when first switch S₁ is closed, or whensecond switch S₂ is closed. Switches S₁ and S₂ are never closed at thesame time. Reference voltage V, is switchably and operably connected tosecond capacitor C₂ by a fourth switch (designated as S_(c) at thebottom right-hand-most switch of FIG. 8, where S_(c) is a controlsignal), and the switched capacitance filtering circuit furthercomprises an output switch (also designated as S_(c), and which is alsoa control signal, but at the top right-hand-most switch of FIG. 8). Thefourth switch and the output switch are configured to close when firstand second switches S₁ and S₂ are open, but after first and secondcapacitors C₁ and C₂ have been charged with the first and secondintegrated signals and the third switch has been closed and opened.

In other embodiments, switched capacitance filtering circuit of FIG. 8may be extended to include additional pairs of capacitors as shown incircuits 64 of FIGS. 12 and 13, where each of switched capacitancefiltering circuits 64 comprises a plurality of pairs of first and secondcapacitors, each pair of first and second capacitors being electricallyconnected in series, the pairs being arranged in parallel with respectto one another, and where only one pair of capacitors is charged duringa single cycle of high and low drive states, and the pairs are chargedwith first and second integrated signals corresponding to sequentialcycles of high and low drive states. The output signals provided by suchswitched capacitance filtering circuits 64 are representative of anaveraged or enhanced signal-to-noise mutual capacitance.

Moreover, touchscreen system 110 or processor 100 preferably comprises areset circuit configured to drain charge remaining on each of theplurality of sense and drive electrodes after a sense cycle has beencompleted. In one the processor is an integrated circuit. By way ofexample, processor 100 may be an integrated circuit that is one of amicroprocessor, a controller, or an application specific integratedcircuit (ASIC), and may be formed using a CMOS or BiCMOS process.

Note that the various teachings presented herein may be applied tooptically transmissive or non-optically-transmissive touchpads disposed,for example, on a printed circuit board, a flex board or other suitablesubstrate. While the primary use of capacitive touchscreen 90 isbelieved likely to be in the context of relatively small portabledevices, and touchpads or touchscreens therefore, it may also be ofvalue in the context of larger devices, including, for example,keyboards associated with desktop computers or other less portabledevices such as exercise equipment, industrial control panels, householdappliances, and the like. Similarly, while many embodiments of theinvention are believed most likely to be configured for manipulation bya user's fingers, some embodiments may also be configured formanipulation by other mechanisms or body parts. For example, thesensing, circuit might be located on or in the hand rest of a keyboardand engaged by the heel of the user's hand. Furthermore, variousembodiments of capacitive touchscreen system 110 and capacitivetouchscreen 90 are not limited in scope to drive electrodes disposed inrows and sense electrodes disposed in is columns. Instead, rows andcolumns are interchangeable in respect of sense and drive electrodes.Various embodiments various embodiment of capacitive touchscreen system110 and capacitive touchscreen 90 are also capable of operating inconjunction with a stylus, such that stylus touches on touchscreen 90are detected. System 110 and touchscreen 90 may further be configured topermit the detection of both of finger touches and stylus touches.

Note further that included within the scope of the present invention aremethods of making and having made the various components, devices andsystems described herein. For example, according to various embodimentsthere is provided a method of reducing electromagnetic interference in acapacitive touchscreen system comprising a touchscreen comprising afirst plurality of electrically conductive drive electrodes arranged inrows or columns, and a second plurality of electrically conductive senseelectrodes arranged in rows or columns arranged at an angle with respectto the rows or columns of the first plurality of electrodes, mutualcapacitances existing between the first and second pluralities ofelectrodes at locations where the first and second pluralities ofelectrodes intersect, such mutual capacitances changing in the presenceof one or more fingers or touch devices brought into proximity thereto,a system ground being associated with the capacitive touchscreen system,an electric field ground being associated with the one or more fingersor touch devices and the touchscreen, the method comprising sequentiallydriving the first plurality of electrodes with alternating high and lowstate drive signals, sensing the mutual capacitances associated witheach of the second plurality of electrodes during the high and low statedrive signals with a charge integrator circuit for each sense circuit,for each charge integrator circuit, providing first and secondintegrated signals corresponding to the high and low state drivesignals, respectively, to a switched capacitance filtering circuit,sampling and storing in first and second capacitors, respectively, ofthe switched capacitance filtering circuit the first and secondintegrated signals, and providing, with the switched capacitancefiltering circuit, an output signal representative of the mutualcapacitance of the one of the second plurality of electrodescorresponding to the sense circuit and further having an electromagneticinterference voltage between the system ground and the electric fieldground substantially filtered therefrom by the switched capacitancefiltering circuit.

Such a method may further include one or more steps of electricallyconnecting the first and second capacitors in series; providing in theswitched capacitance filtering circuit first and second input switchescorresponding to the first and second capacitors, respectively;configuring the first input switch to close and charge the firstcapacitor when the first integrated signal corresponding to the highstate drive signal is delivered thereto while the second switch is open;configuring the second input switch to close and charge the secondcapacitor when the second integrated signal corresponding to the lowstate drive signal is delivered thereto while the first switch is open,switchably and operably connecting a reference voltage between the firstand second capacitors by a third switch; configuring the third switch toclose and provide the reference voltage to the switched capacitancefiltering circuit when the first or second switch is closed; switchablyand operably connecting the reference voltage to the second capacitor bya fourth switch and providing the switched capacitance filtering circuitwith an output switch; closing the fourth switch and the output switchwhen the first and second switches are open, but after the first andsecond capacitors have been charged with the first and second integratedsignals and the third switch has been closed and opened; configuring theswitched capacitance filtering circuit such that it comprises aplurality of pairs of first and second capacitors, each pair of firstand second capacitors being electrically connected in series, the pairsbeing arranged in parallel with respect to one another; charging onlyone pair of capacitors during a single cycle of high and low drivestates, the pairs being charged with first and second integrated signalscorresponding to sequential cycles of high and low drive states; andproviding an output signal from the switched capacitance filteringcircuit that is representative of an averaged or enhancedsignal-to-noise mutual capacitance.

The above-described embodiments should be considered as examples of thepresent invention, rather than as limiting the scope of the invention.In addition to the foregoing embodiments of the invention, review of thedetailed description and accompanying drawings will show that there areother embodiments of the present invention. Accordingly, manycombinations, permutations, variation and modifications of the foregoingembodiments of the present invention not set forth explicitly hereinwill nevertheless fall within the scope of the present invention.

1. A capacitive touchscreen system, comprising: a touchscreen comprisinga first plurality of electrically conductive drive electrodes arrangedin rows or columns, and a second plurality of electrically conductivesense electrodes arranged in rows or columns arranged at an angle withrespect to the rows or columns of the first plurality of electrodes,mutual capacitances existing between the first and second pluralities ofelectrodes at locations where the first and second pluralities ofelectrodes intersect, such mutual capacitances changing in the presenceof one or more fingers or touch devices brought into proximity thereto;at least one drive circuit operably connected to the first plurality ofelectrodes and configured to provide drive signals of high and lowstates thereto; a plurality of sense circuits, one each of the sensecircuits being operably connected to corresponding ones of the secondplurality of electrodes; a system ground associated with the capacitivetouchscreen system; an electric field ground associated with the one ormore fingers or touch devices and the touchscreen; wherein each of thesense circuits comprises a charge integrator circuit and switchedcapacitance filtering circuit, the charge integrator circuit beingconfigured to receive input signals provided by a corresponding one ofthe second plurality of electrodes and to provide integrated signals tothe switched capacitance filtering circuit, the switched capacitancefiltering circuit being configured to sample and store in first andsecond capacitors, respectively, first and second integrated signalsdelivered thereto by the charge integrator circuit where the firstintegrated signal corresponds to the high state drive signal, and thesecond integrated signal corresponds to the low state drive signal, theswitched capacitance filtering circuit being configured to provide anoutput signal representative of the mutual capacitance of the one of thesecond plurality of electrodes corresponding to the sense circuit andfurther having an electromagnetic interference voltage between thesystem ground and the electric field ground substantially filteredtherefrom by the switched capacitance filtering circuit.
 2. Thecapacitive touchscreen system of claim 1, wherein the first and secondcapacitors are electrically connected in series.
 3. The capacitivetouchscreen system of claim 2, wherein the switched capacitancefiltering circuit comprises first and second input switchescorresponding to the first and second capacitors, respectively.
 4. Thecapacitive touchscreen system of claim 3, wherein the first input switchis configured to close and charge the first capacitor when the firstintegrated signal corresponding to the high state drive signal isdelivered thereto while the second switch is open.
 5. The capacitivetouchscreen system of claim 4, wherein the second input switch isconfigured to close and charge the second capacitor when the secondintegrated signal corresponding to the low state drive signal isdelivered thereto while the first switch is open.
 6. The capacitivetouchscreen system of claim 5, wherein a reference voltage is switchablyand operably connected between the first and second capacitors by athird switch.
 7. The capacitive touchscreen system of claim 6, whereinthe third switch is configured to close to provide the reference voltageto the switched capacitance filtering circuit when the first or secondswitch is closed.
 8. The capacitive touchscreen system of claim 7,wherein the reference voltage is switchably and operably connected tothe second capacitor by a fourth switch, and the switched capacitancefiltering circuit further comprises an output switch.
 9. The capacitivetouchscreen system of claim 8, wherein the fourth switch and the outputswitch are configured to close when the first and second switches areopen, but after the first and second capacitors have been charged withthe first and second integrated signals and the third switch has beenclosed and opened.
 10. The capacitive touchscreen system of claim 1,wherein the switched capacitance filtering circuit comprises a pluralityof pairs of first and second capacitors, each pair of first and secondcapacitors being electrically connected in series, the pairs beingarranged in parallel with respect to one another.
 11. The capacitivetouchscreen system of claim 10, wherein only one pair of capacitors ischarged during a single cycle of high and low drive states, and thepairs are charged with first and second integrated signals correspondingto sequential cycles of high and low drive states.
 12. The capacitivetouchscreen system of claim 11, wherein the output signal provided bythe switched capacitance filtering circuit is representative of anaveraged or enhanced signal-to-noise mutual capacitance.
 13. A processorfor a capacitive touchscreen system comprising a touchscreen comprisinga first plurality of electrically conductive drive electrodes arrangedin rows or columns, and a second plurality of electrically conductive,sense electrodes arranged in rows or columns arranged at an angle withrespect to the rows or columns of the first plurality of electrodes,mutual capacitances existing between the first and second pluralities ofelectrodes at locations where the first and second pluralities ofelectrodes intersect, such mutual capacitances changing in the presenceof one or more fingers or touch devices brought into proximity thereto,a system ground being associated with the capacitive touchscreen system,an electric field ground being associated with the one or more fingersor touch devices and the touchscreen, the processor comprising: at leastone drive circuit operably connected to the first plurality ofelectrodes and configured to provide drive signals of high and lowstates thereto; a plurality of sense circuits, one each of the sensecircuits being operably connected to corresponding ones of the secondplurality of electrodes; wherein each of the sense circuits comprises acharge integrator circuit and a switched capacitance filtering circuit,the charge integrator circuit being configured to receive input signalsprovided by a corresponding one of the second plurality of electrodesand to provide integrated signals to the switched capacitance filteringcircuit, the switched capacitance filtering circuit being configured tosample and store in first and second capacitors, respectively, first andsecond integrated signals delivered thereto by the charge integratorcircuit where the first integrated signal corresponds to the high statedrive signal, and the second integrated signal corresponds to the lowstate drive signal, the switched to capacitance filtering circuit beingconfigured to provide an output signal representative of the mutualcapacitance of the one of the second plurality of electrodescorresponding to the sense circuit and further having an electromagneticinterference voltage between the system ground and the electric fieldground substantially filtered therefrom by the twitched capacitancefiltering is circuit.
 14. The processor of claim 13, wherein the firstand second capacitors are electrically connected in series.
 15. Theprocessor of claim 14, wherein the switched capacitance filteringcircuit comprises first and second input switches, corresponding to thefirst and second capacitors, respectively.
 16. The processor of claim15, wherein the first input switch is configured to close and charge thefirst capacitor when the first integrated signal corresponding to thehigh state drive signal is delivered thereto while the second switch isopen.
 17. The processor of claim 16, wherein the second input switch isconfigured to close and charge the second capacitor when the secondintegrated signal corresponding to the low state drive signal isdelivered thereto while the first switch is open.
 18. The processor ofclaim 17, wherein a reference voltage is switchably and operablyconnected between the first and second capacitors by a third switch. 19.The processor of claim 18, wherein the third switch is configured toclose to provide the reference voltage to the switched capacitancefiltering circuit when the first or second switch is closed.
 20. Theprocessor of claim 19, wherein the reference voltage is switchably andoperably connected to the second capacitor by a fourth switch, and theswitched capacitance filtering circuit further comprises an outputswitch.
 21. The processor of claim 20, wherein the fourth switch and theoutput switch are configured to close when the first and second switchesare open, but after the first and second capacitors have been chargedwith the first and second integrated signals and the third switch hasbeen closed and opened.
 22. The processor of claim 13, wherein theswitched capacitance filtering circuit comprises a plurality of pairs offirst and second capacitors, each pair of first and second capacitorsbeing electrically connected in series, the pairs being arranged inparallel with respect to one another.
 23. The processor of claim 22,wherein only one pair of capacitors is charged during a single cycle ofhigh and low drive states, and the pairs are charged with first andsecond integrated signals corresponding to sequential cycles of high andlow drive states.
 24. The processor of claim 23, wherein the outputsignal provided by the to switched capacitance filtering circuit isrepresentative of an averaged or enhanced signal-to-noise mutualcapacitance.
 24. The processor of claim 13, further comprising a resetcircuit configured to drain charge remaining on each of the secondplurality of electrodes after a sense cycle has been completed.
 25. Theprocessor of claim 13, wherein the processor is an integrated circuit.26. The processor of claim 25, wherein the integrated circuit is one ofa microprocessor, a controller or an application specific integratedcircuit (ASIC).
 27. The processor of claim 25, wherein the integratedcircuit is formed using a CMOS or BiCMOS process.
 28. A method ofreducing electromagnetic interference in a capacitive touchscreen systemcomprising a touchscreen comprising a first plurality of electricallyconductive drive electrodes arranged in rows or columns, and a secondplurality of electrically conductive sense electrodes arranged in rowsor columns arranged at an angle with respect to the rows or columns ofthe first plurality of electrodes, mutual capacitances existing betweenthe first and second pluralities of electrodes at locations where thefirst and second pluralities of electrodes intersect, such mutualcapacitances changing in the presence of one or more fingers or touchdevices brought into proximity thereto, a system ground being associatedwith the capacitive touchscreen system, an electric field ground beingassociated with the one or more fingers or touch devices and thetouchscreen, the method comprising: sequentially driving the firstplurality of electrodes with alternating high and low state drivesignals; sensing the mutual capacitances associated with each of thesecond plurality of electrodes during the high and low state drivesignals with a charge integrator circuit for each sense circuit; foreach charge integrator circuit, providing first and second integratedsignals corresponding to the high and low state drive signals,respectively, to a switched capacitance filtering circuit; sampling andstoring in first and second capacitors, respectively, of the switchedcapacitance filtering circuit the first and second integrated signals;providing, with the switched capacitance filtering circuit, an outputsignal representative of the mutual capacitance of the one of the secondplurality of electrodes corresponding to the sense circuit and furtherhaving an electromagnetic interference voltage between the system groundand the electric field ground substantially filtered therefrom by theswitched capacitance filtering circuit.
 29. The method of claim 28,wherein the first and second capacitors are electrically connected inseries.
 30. The method of claim 29, wherein the switched capacitancefiltering circuit comprises first and second input switchescorresponding to the first and second capacitors, respectively.
 31. Themethod of claim 30, wherein the first input switch closes and chargesthe first capacitor when the first integrated signal corresponding tothe high state drive signal is delivered thereto while the second switchis open.
 32. The method of claim 31, wherein the second input switchcloses and charges the second capacitor when the second integratedsignal corresponding to the low state drive signal is delivered theretowhile the first switch is open.
 33. The method of claim 32, wherein areference voltage is switchably and operably connected between the firstand second capacitors by a third switch.
 34. The method of claim 33,wherein the third switch closes to provide the reference voltage to theswitched capacitance filtering circuit when the first or second switchis closed.
 35. The method of claim 34, wherein the reference voltage isswitchably and operably connected to the second capacitor by a fourthswitch, and the switched capacitance filtering circuit further comprisesan output switch.
 36. The method of claim 35, wherein the fourth switchand the output switch close when the first and second switches are open,but after the first and second capacitors have been charged with thefirst and second integrated signals and the third switch has been closedand opened.
 37. The method of claim 28, wherein the switched capacitancefiltering circuit comprises a plurality of pairs of first and secondcapacitors, each pair of first and second capacitors being electricallyconnected in series, the pairs being arranged in parallel with respectto one another.
 38. The method of claim 37, wherein only one pair ofcapacitors is charged during a single cycle of high and low drivestates, and the pairs are charged with first and second integratedsignals corresponding to sequential cycles of high and low drive states.39. The method of claim 38, wherein the output signal provided by theswitched capacitance filtering circuit is representative of an averagedor enhanced signal-to-noise mutual capacitance.